docs: def variables(self) -> list:
          """Variables."""
          try:
              assert self._variables is not None
          except (AssertionError, AttributeError):
              pattern = "|".join(map(re.escape, operators))
              keys = re.split(pattern, self.expression)
              indices = []
              for key in keys:
                  if key in self.parent.variable_names:
                      indices.append(self.parent.variable_names.index(key))
              self._variables = [self.parent.variables[i] for i in indices]
          finally:
              return self._variables